Nested tuple argument unpacking.

def f((a,b), c):
pass

This was removed in Python 3.

Function attributes

Though it's generally transparent to a Python user, Python has several different kinds of function objects; in particular one for C-defined functions (a.k.a. function pointer wrappers) and one for Python-defined functions (a.k.a. bundles of bytecodes):

inspect support

While it is quite possible to emulate the interface of functions in Cython's own function type, and recent Cython releases have seen several improvements here, the "inspect" module does not consider a Cython implemented function a "function", because it tests the object type explicitly instead of comparing an abstract interface or an abstract base class. This has a negative impact on code that uses inspect to inspect function objects.

Probably won't fix

This is a list of things that fall more into the implementation details rather than semantics, and we may decide not to fix (or require a --pedantic flag to get).

Stack frames: Currently we generate fake tracebacks as part of exception propagation, but don't fill in locals and can't fill in co_code. To be fully compatible, we would have to generate these stack frame objects at function call time (with a potential performance penalty).